It's doable, but 2" W material would be better.
For reference, this is what my current spacers look like:
It's constructed of a piece of 2.5" sq. tube x 2.5" H x .250" wall, with 5.5" L x 2.5" W x .25" pads to make a 3" spacer (the height is irrelevant for this discussion, though; the 2.5" W base is what matters here). You could easily just use a 2.5" x 2.5" x 5.5" tube to replicate this like other spacers, normally 2" W x 4" H x 6" L for some of the traditional spacers. I did 5.5" L spacers to avoid having to notch anything for the passenger side where the brake lines come down (the metal guard shown in the pic).
Here are the gotchas:
1. A 2.5" wide spacer is just a tad too wide. On the rear subframe, the width is perfect relative to the chassis. But, the unseen lower edge of the spacer actually sits atop a weld on the subframe, like so:
For the rear spacers, you could obviously offset the holes on the spacer to get around that, but the spacer wouldn't sit flush. Also, if you do a simple tube approach, you have the radius edges to your advantage to some degree, but it may still be wide.
2. The front spacer is a little similar, but you can't offset the holes because of the lip on the chassis:
Although this fits flat, it does barely contact similar weld on the subframe, it doesn't give much wiggle room for installation. The radius edge of the tube would help here.
3. A 2.5" sq. tube, presumably .250" if hitch steel, only leaves 2" of tool space inside the tube for tightening hardware. It's doable, but just be prepared to make a lot of small motions to get the upper hardware tightened. And if you orient the lower bolts tip-down (which you should), be sure to put those bolts in sometime before sandwiching things together, otherwise you might not have the clearance to insert the bolt or upper nut.
That all said, eventually I will go to spacers with a 2"W base to get around those issues, or possibly convert these by welding on a 2"W x .25"H plate to the bottom of each spacer to clear the subframe welds (resulting in a 3.25" spacer).