I have few underwear wear from sukrew brand, they are great specially their thong,
They had some with open at the front a while ago so you might have to look online for the brand.
I one and it’s very sexy
Have fun
https://www.sukrew.com/u-thong-moonlight.html
Try on Ebay: UK Men's Thong G-String Faux Leather Briefs Jockstrap T-back Backless Underwear
or on Ebay:
K017 Hot Men Sexy String Thong Pouch w/ Front Open
K017 HOT Herren Sexy String Tanga Pouch mit vorne offen
( 180716362016 )
swirlfashion19138
Koala has a bunch of exciting options - their gear is always on the risqué end of the market!
https://www.koalaswim.com/Diablo-Insane_p_132.html
That’s just one...click on ‘display suits’ on the site...
Let me know what you think
I know a guy based in the UK who makes phones of all types from Lycra to order. His prices are reasonable and he does a great job. He would definitely be able to create you something for a fraction of the price with an open front.