This is an inexpensive way to stiffen your printer.
You have probably printed infill on small parts, or using a hexagonal pattern and been dismayed by how much the top of your printer was flexing and swaying!
This will make your printer's frame amazingly stiff, and only a little bit heavier -- triangles are much stronger than rectangles.
You will print out one of these (which contains four side posts) for each side of your printer you wish to stiffen.
You will need twelve M3x10 screws, nine M3 nuts, and one M3x8 screws (four of the M3x10 screws will thread into plastic).
Get two 36" pieces of 1/2"x1/16" (or 1/2"x1/20") aluminum L-channel from Home Depot (or your favorite hardware store) for each side. At my HD, a 36" piece is $2.36, and you can also buy it in 8' lengths.
Insert four captive nuts into outside face of the top extrusion, and another four into the outside face of the bottom extrusion.
Cut the two 36" pieces of aluminum L-channel down to 810mm long.
First prepare to install the two shorter posts on opposite corners of one side of your printer, with the angle in the L-channel facing toward your printer. Drill the 3mm (or 1/8") holes near the ends of the L-channel and dry fit them. Then attach this piece to your printer using four M3x10 screws, and attach the aluminum to the side posts using two more M3x10 screws. Do this carefully so that the aluminum is supporting the printer in the exact position your printer was without it -- you don't want your supports to twist your printer!
I first loosely attached the side posts so that they could still slide, then I loosely attached the aluminum to them, and then I tightened the aluminum, and lastly tightened the side posts.
Second prepare to install the two taller posts on the other two opposite corners of the same side of your printer, this time with the angle in the L-channel facing away from your printer. Same procedure as before.
Third, at the point where the two pieces of aluminum cross (with their flat sides together), drill one more hole and install the M3x8 and the last nut.
Last night I used two 760mm pieces of aluminum L-channel to stiffen the front side of my printer (and still leave a large opening to remove objects from). It is shown in the last image. It uses three copies of smallerSidePost.stl, plus the expected M3 screws and nuts.