[Bruno]

The most simple solution is to just use the UltraTech armor, and the usual GURPS rules for equipment (Basic Set p27, "Tech Level and Equipment" is the big important box here).

I'll make some assumptions for a worked example: Your game is a TL 4+2 clockpunk world with big "thinking engines" and wind-up trains and dirrigables and whatever; that's TL 6-y. You want an Infantry Combat Walker (TL9, $300,000).

Having a TL 9 item at TL 6 is 8x the base cost - this is ridiculously expensive, so you start looking for corners to cut, and the low-hanging fruit are accessories that are hard to justify at clockpunk. Pull out the biomedical sensors (-200), hyperspectral visor (-2000), and laser comm (-400), along with the ESM (-1000), infrared cloaking (base 1500 but adjusted for SM 2 for -7500), and radar stealth (again, base 1500 adjusted to SM 2 for -7500).

This takes 18, 600 off the base cost, and where we are every dollar counts. Finally, make it Cheap, per UT15, which halves cost, but multiplies weight by 1.5; in this context it's easy to understand, a hyper-efficient steam generator and a coal bin need to be strapped on this thing, after all :P
In this context, I'd say the extra 400 lbs count as non-removable encumbrance for the Walker. A clockpunk walker should be less nimble.

So, $300,000 - $18,600 = $281,400. Halve for Cheap, $140,700. Final step, adjust for "out of TL" x8 cost for $1,125,600.

As a major technomagic item that makes you some sort of terrifying siege golem, that's not bad. You almost certainly need a caddy following in your wake, frantically shoveling more coal in.

Adjust the final step as per usual depending on your local TL, classical "Dungeon Fantasy" tends to oscillate between effective TL 3 to effective TL 6, depending on how common magic is. Since you're talking about powered armor, I assumed more on the common side :)