I already have a Rodeoh, and I've used that for packing before, but of course I can't use that more than one day in a row (two or maybe three if I use pantiliners). I haven't tried using a makeshift strap under normal underwear yet, but that's my next step. In an ideal world, I'd have enough packing underwear to get through the week, but that'll take awhile on my budget.
I would have said the trunks ones, but after reading the posts here it seems better to DIY one or use a harness or jockstrap. Except if you intend not to wear it too often or wear other underwear underneath, seems better for hygiene.
i would make a DIY one similar to the rodeoh. or make a couple like that, even. and wear them outside briefs if you're worried they'll get nasty. that or homemade straps (not an option for me because mine's an STP device). all kinds of info on diy harnesses/straps on tumblr
I like the SpareParts Pete trunks packer (M) the best.
If you want something minimalist, you can sew a loop of thick elastic to fit where you want the packer to hit on your body, and then make a small horizontal slit at the front. wedge the shaft through the slit and there you go. an old underwear band with the rest cut off will work too.
Actually, my boyfriend and I find it to be much cheaper to just make him his own. Get some cotton boyshorts (yes sadly you have to use ladies underwear because they're more form-fitting and briefs don't quite cut it). Find a plastic (metal will hurt you and you wont be able to wash them) O-rings in your size. Cut a small hole where you feel it would be comfortable and sew the O-ring into it. Make sure it is very secure. Cut out a piece of a shirt or other material soft enough to be comfortable on your skin and form it to the crotch area of the underwear. Sew that on tightly, leaving a fair amount of room for your balls and an opening at the top. It's up to you what color you'd like but nude is what he prefers since it's like a second skin. I hope this helps some