The best of both worlds- trunks and swim briefs?

[speedough]

By square cut, I mean http://www.aussiebum.com/swimwear/freshhh-break/591. These have 6" sides and are probably a little smaller than a Sunga which would be worse. I have these and they just don't support like the briefs do.

[Torchwatch]

I's the leg hole elastic loops that give speedos their support.
Sungas and squarecuts have leg hole elastic looks further away from the support area.
Some expensive jammers have elastic shaping the support area like speedos although they go down to the knees.
You seem to want a square cut with "speedo leg loops" built into the design.
It's possible to make but possibly messy.

[Swimmboy]

You might try a simple pair of bikini briefs under the trunks - you get the support you want but still look great!

[Hotbunz1969]

Why not try a well fitted thong or g-string under your square cuts, that way you'll have support up front and no double material in the rear.