Two days ago I installed a MAXXHAUL Swing-back trailer jack (Amazon for like $35 ). Its bolt-on, 1500 lb capable with the dual plastic wheels. Harbor Freight has a similar version.
My M103-A3 trailer is still a work in progress, so its probably not in the final location, but at least its usable-ish. I mounted it on the driver's side inside the "V" of the tongue. It could be mounted on the outside but the big grab handle is in the way. The bolt on jacks are meant for 5" frames and these trailers are maybe 1/8" over that, so I ground a flat on the shoulder of 2 of the long bolts. One option/plan when I redo this is to get 1" longer bolts and grind flats on all 4 to make assemble easier.
Option 2 (ultimate!) is to get the weld-on version with round collar. I would then weld 2" receiver tube under the tongue and in the back corners of the trailer. Weld the round collar to male 2" tube and I can move it around as needed, or get extra couple and be able to level the trailer. I will then make a male receiver mount for my cheapo winch and be able to put it anywhere on the trailer for pulling logs, etc.