If you want to convert the stock setup with a pretty small amount of modding and cost, I think the best way is to buy 2 of
Watt-man's bypass fittings with hose barbs (or similar). Splice one into the upper hose between the engine and thermostat housing. But swap the hose barb and temp sender, so the sender is in the new fitting, and the hose barb is in the stock stat housing. This keeps the temp sender in the coolant flow and puts the bypass at the thermostat. Splice the second bypass fitting into the lower radiator hose as usual and connect the two barbs with the bypass hose.
Caveats- You'll have to run a ground wire to the new bypass fitting for the temp sender to work properly there.
- I don't know what's around those parts on the 250s. Certain angles or sizes of hose barbs may not actually fit in those places on the bike.
- I don't know what size/thread Watt-man's hose barbs use. I'm pretty sure the EX500's temp sender is 1/8" BSP, but the 250s may be different. He may use 1/4" and/or NPT instead. This means it might not be as simple as "unscrew them and put them in the other holes" to swap. You may need to buy different barbs and/or some adapters and find a way to make it all fit (similar to the EX500 DIY).
- Ideally with an open bypass system like this, you want the input and radiator legs to be a straight shot, and the bypass to be a right-angle to the side (like the TB's design). This helps the coolant flow prefer the radiator over the bypass when the stat is open. In the stock housing, the temp sender port is kind of straight-through, while the radiator path is turned up. However, the sender port is much smaller, and the thermostat itself interrupts the flow to it, so it shouldn't be a huge deal.
- With the improved flow of a bypass system, the coolant in the engine shouldn't really be any hotter than the coolant at the thermostat (unlike the stock system with restricted flow). The stock 180°F stat might be cooler than ideal after the conversion. You might want to try to find a ~195°F replacement stat.
If you don't mind spending a bit on the full Thermo-Bob setup, it should fit in place of the stock thermostat housing without too much work. Make sure the TB is grounded and transfer the temp sender over, splice the bypass return into the lower radiator hose, and you're done. There's more cost, but there's less work, you get a better stat housing design, and the hotter stat is included.
If you can pull off the DIY for $40, that's quite a savings. If it ends up being $100 for miscellaneous parts you have to cobble together, you might as well just spend the extra $30 and buy the TB as a quality, complete product.