Spray rig I made under $100

[Eugene]

I use alligator clips attached to the tractor's battery and a toggle switch to control my sprayer. My out of pocket cost, nothing, parts from junk box.

[Mht]

A couple more pieces of wood from the tongue near the hitch to the outer ends of the cross piece just above the vertical pieces where the wheels are attached would add a tremendous amount of resistance against twisting apart should a wheel hit a rock or something without hampering ground clearance. You could maybe add a diagonal brace from that to the vertical leg the wheels are mounted on which would keep the wheels from folding back if they hit an obstruction without losing too much clearance. Just my thoughts from an old school carpenter who made a lot of things from wood before I got a welder

[BullDAWG]

Before you go hard wiring a bunch of stuff, take a look at this switch. You can find cheaper ways to do this, but few are more convenient.
https://www.tractorsupply.com/tsc/product/fimco-12-volt-wireless-remote-on-off-switch

Here's my "cheaper solution" that ended up being $25. The above would have been a more elegant solution.
Sealed Blade fuse holder $4
https://www.autozone.com/electrical-and-lighting/fuse-and-accessories/bussmann-atc-fuse-holder-with-cover/32415_0
Single Pole Single Throw Switch $5
https://www.autozone.com/electrical-and-lighting/toggle-switch/dorman-conduct-tite-chrome-35-amp-toggle-switch/6450_0_0
Panel Mount Cigarette Lighter $11
https://www.amazon.com/attwood-11862-6-12V-Marine-Outlet/dp/B003EET3XY/ref=pd_cp_263_4/138-4700121-9643729?_encoding=UTF8&pd_rd_i=B003EET3XY&pd_rd_r=fa48e6b4-49fa-42ab-ab6e-938738bcaa71&pd_rd_w=ffeDl&pd_rd_wg=l6c6V&pf_rd_p=0e5324e1-c848-4872-bbd5-5be6baedf80e&pf_rd_r=3ECD077BPXFR2TKRZ50K&psc=1&refRID=3ECD077BPXFR2TKRZ50K
Male end for the Cigarette lighter - This extension cord was cheaper than just buying a single end. $5
https://www.amazon.com/TOOGOO-Accessory-Cigarette-Lighter-Extension/dp/B06Y5F1SPK/ref=sr_1_1?crid=3ADO192FU5D0R&keywords=cigarette+lighter+extension+cord&qid=1575895857&s=automotive&sprefix=cigare%2Cautomotive%2C183&sr=1-1

After all that, I wish I had used the top solution or something like the below that has it all together in one package at ace hardware.
https://www.acehardware.com/departments/lawn-and-garden/outdoor-power-equipment/pressure-washer-accessories/7527369?x429=true&utm_source=google&utm_medium=cpc&gclid=Cj0KCQiA_rfvBRCPARIsANlV66MRjRp759USNN3S9AL1vjN8DbqQBw2oGDE4TpXjyEi7ezOBW7ksns4aAmlPEALw_wcB&gclsrc=aw.ds

I'm like Eugene and had most of those things sitting in leftovers toolbox/spare parts. I do like the remote though and the factory switch is like the last part from ace hardware. I just removed the alligator clips, added a fuse to hot side and then put on crimped on spades that fit in the battery cable clamps bolts. That remote though could just be added easily as the factory cable has the same type of connections as it has and would just go in line.

Mht, I like the idea of adding some cross braces angled @ 45 deg going from vertical legs to the tongue. I don't know if going all the way to the end though may be best as I think that would limit turning radius as they may hit the rear tractor tires then. But just having them back 18" in from the hitch should clear tires fine then and still give extra support to the vertical legs.