I've just pasted 2 layers of overexposed film negatives over a Hoya Skylight 1B filter. I've cropped 3 negative strips for each layer to cover the entire filter and taped it in place in the borders. The second layer is perpendicular to the first one to avoid visible light leaking between the negative strips.
Someone mentioned a similar design in an IR forum, but he was clever than me, and instead of pasting filmstrips, he used 2 filters and just sandwiched the filmstrips betweem them. Of course you need 2 filters instead of one, and some vignetting can occur in superwideangles, but it's a more "elegant" solution.
A second option, if you don't have a filter to play, is to paste a single strip per layer directly in the plastic surrounding the front element. This have two problems: first, you are taping things directly to the lens, and that's not a very good idea, even in a $100 lens; second, as you need to remove the filter to be able to focus and compose, the filter must be removed and reattached in every picture, and it's not the same to screw and unscrew a filter than detaching and attaching some film strips with tape every time (including the consequent risk to the lens every time you do the procedure).
Of course, the best idea is to buy an IR filter (say "expensive", please), but to test the camera capabilities for no money, and to have a taste of IR photography, this is a good start
Some pictures of my "filter"
The original Hoya 1B and one "layer" of the filter:
The Hoya with the first layer in place:
The 18-55 with one strip pasted directly to the lens (NOT RECOMENDED, I've done that to take the picture only):
Well, enjoy your homemade filter