I need help with something to anyone that has experience in underwear or panty filling before of any kind.
Basically, I was wondering what would you recommend to buy that will help keep any kind of substance or food inside your underwear without anything escaping from the legholes? I only done this a few times now and while I do enjoying the feeling of this experience, I wanna try to keep everything in and not have anything fall out but I don't know where to start looking for what I want.
If you have any suggestions and recommendations for what to buy I'm all ears.

I got some cycling shorts that are very tight and dont seem to leak that much

You could wear multiple layers... Briefs/trunks or boxers, and then longer lycra shorts over the top. Stuff may start squidging down your leg eventually, depending on how much activity you're doing, but may not actually emerge beyond the shorts!

What you're after are gym knickers. They have elastic at the leg holes as well as at the waist, so hold more in, and won't leak, at least at first. Once they soak through they will, but by liquid coming through the fabric, rather than leg hole leaks.

Speaking from experience the only pants you need to be googling is

https://www.capatexcare.co.uk/drytex-waterproof-pvc-plastic-pants

If you can find pvc inconvenience pants they ain't gonna let anything out infact I did a whole zumba class with a trifle inside

https://umd.net/download_info/naughty-housewife-pvc-pants-zumba-experi

this is a good one, I never tried on plastic pants before but maybe this could work. I'm a little mixed on it being see-through but I think I can maybe put it over a another pair of underwear. thank you for this suggestion!!

this is a good one!! I never thought of gym underwear before but that could definitely work. I might see if they make one for men or if it's only for females which if it is I'll probably still get them regardless. thank you for the recommendation!!

I have used plastic adult baby pants before with no leaks

You can buy white ones that are not see through.

You can buy white ones that are not see through.

Search ebay for 'black plastic pants' there are numerous types

So update: I searched a bit and found some options I'm interested in messing in. Here's 3 I found:
1. https://girlfriend.com/products/shell-mid-rise-brief
2. https://www.shethinx.com/collections/speax-core-collection/products/speax-hiphugger?variant=29209484787784
3. https://knix.com/products/essential-boyshort?clickId=3985320314&gclid=CjwKCAjwq57cBRBYEiwAdpx0vbTn-gRiLW1IO3_okQ3w17HWj6trO_ompH13IWqA8JM2gLdtDfw7DxoCPC4QAvD_BwE&utm_campaign=120661&utm_medium=affiliate&utm_source=pepperjam&variant=736306888710

I don't know if these 3 will hold as much but I'm try and if not I'll double layer. I also forgot that absorbent panties are a thing which might be fun to test out with liquids like water for example. I also don't know if the size of women's underwear are different then men's like I'm a 2XL in men's so I'm not sure if I should get the same size in women's or something smaller depending on the shape of them.

I'm currently planning on what to fill these panties with as I have many ideas like maybe filling it with whip or shaving cream or lot's of food condiments, slime, water balloons that I fill up while in my underwear and even potentially expanding foam (I was considering getting that but I think some might be toxic and not a good idea to fill my underwear with but just the thought of foam inside undies that expand on their own is amazing.)

Let me know if the three panties options are good enough and which one I should get. Also if there is expanding foam available to purchase that is safe to use and not toxic please let me know as soon as possible cause I would wanna try that!!

Lisa's my girl!!!!!!!!!!!!!!!!!

Wetsuit shorts also work pretty well. Many have elastic around the legs, and will not let much mess out in my experience.
However, they aren't cheap, and will eventually soak through.
Loving Cinq's recommendations - I reckon I'll give that a try myself!