I don't know what style device you plan to diy, but I built my first version of the ClutchTamer years ago for about $20 using hardware store materials...
The Hillbilly Slipper is amazingly effective, but the cylinder itself is about 13" overall and it's internal spring is stiff enough to add about 25lbs of effort using 3.25" of it's stroke. Here's a link to more info...
DIY Hillbilly Clutch Slipper Install
Here's a simple diy ClutchMasters/Tilton style one-way simple restriction valve that you can build with a spring, nitrous jet, and some hardware store fittings. The nitrous jet is seated against the inlet with a spring, the inlet fitting is chamfered slightly so the jet can seat against it.
...when the fluid travels from the master cylinder, the jet get's blown off it's seat and fluid bypasses the orifice.
...when the fluid travels from the slave to the master, the spring and fluid seat the jet against the inlet fitting, which forces return fluid to travel through the orifice.
Combine it with a solenoid bypass, you will have what's basically a MAP/Magnus style system without the dial adjustment.
If you want the dial style adjustment, just substitute the above diy valve with a Parker F200S Flow Control Valve. Resolution isn't great at the amount of restriction you will need, but it's the same valve used by MAP/Magnus...
If you want a diy version of my Hitmaster valve, i'm not going to help you with that as it is currently "patent pending"
Grant