Similar to Roper Whitney Hand Punch

A hobbyist would typically use a nibbler to make non-circular holes.
For those with the dough, there are bench presses with tooling for punching square, rectangular, D, double-D, D-sub, etc. holes. For example, look for the 3000 lb press and matching tooling at
Tim.

I regret to say that I own a knockoff, having felt cheap when I bought it a long time ago. However, I have looked at a real No. 5 and my guess is that Whitney dies will fit in the knockoff imports. You don't get square or rectangular dies with the standard No. 5 anyway, so you'd still have to buy the punch/die sets.
Where are those available?
GWE

Keywords: Roper Whitney sells a slightly larger model (the "XX") that can use non-round dies. It comes with a completely useless & bizarre set of punches, and the punch & die sets are pricey.
Doug White

Allied Electronics used to handle them - I think they are still alive after Tandy.
Other electronic supply houses likely do as well. Wonder if Tandy does... Digikey ? Martin

A shop I once worked in had a punch similar to this fabricating sheet metal electronic chassis parts. The die holder was different -- just 4 horizontal setscrews which positioned and held the bevel-sided lower die. It took me a while to figure out how to set the die quickly and accurately but it was a very useful tool for its size.
My favorite way to make holes in sheet metal is to find a shop with a Strippit, get a list of the punch sizes they own, and design accordingly.
jw

I bought XX punches and dies from MSC. jw

Mouser(electronics wholesaler) has a table-top arbor-type press with that type of tooling. Pump up your wallet, though. I don't know of any Jr-sized hand punches that have square tooling, you'd need something to keep the punch and die aligned radially and they have no provisions for that, arbor press time.
A nibbler has always been my choice for rectangular holes in panels, use Jr to make the first hole, punch the corners, then nibble away and file to the line. Greenlee has square chassis punches, but you won't find them in smaller sizes. If I need a really small squared hole, I'll punch a round one and file to the line with needle files. In the sizes Jr covers, that's not a big deal, maybe 5 mins. worth of work.
Stan