I shoot often in my small living room, and the spill from the 60cm (24") softbox is unbearable. AFAIK, there's no grid of this size available to UK customers in the price range I'd consider buying it.
The Strobist approach, making a grid out of cardboard, doesn't really appeal to me. I'm not going to build a grid each time I'll need one, and I plan on using one often. If I'm to make myself a grid, I want it to be good looking, sturdy and easily portable.
I've tried sewing 50mm black elastic band into a grid. No way, after a few hours I had done only about 10 cells, and even those weren't as regular as to ressemble anything. So I've abandoned that idea.
The next idea was to make a grid out of gaffer's tape (I chose the cheaper Gaffa Tape, 50mm x 50m from Boyes). I've decided to build it in it's collapsed form, otherwise I would have needed some sort of very complex rig to give it shape. I've failed miserably at my first try - I succombed to the urge to check, while building it, if it looks nice. Big mistake - that glue, when it comes into contact with itself, bonds instantly, for good. If you mess up, you shouldn't waste time trying to unglue and rearrange it - just throw it all away, take some time to remember what your uncle Bob used to say when he crushed his thumb with a hammer, then start over.
I did start over. The concept is stupid simple. Using a cardboard template cut to the required size, make rings out of tape, sticky side out (1). Stick them on a smooth working surface, they will come out easily (2). Remove the template and press flat the cell with your finger. Stack the cells to the right half-length apart to make a row (3), in my case a row would be 10 cells long. Start the following row half-length to the left, carefully aligning the current cell's right edge to the left edge of the second cell on the row below (no pic, sorry). Keep rolling and laying cells for as long as it takes. Don't give up, don't rush it, and DO NOT unfold the grid until it's finished and no sticky surfaces are left uncovered. If you carefully align all the edges on all the rows, the structure unfolds as a pretty regular grid (4). You only need to add some velcro on it's edges (I've put the fluffy side on the grid; I'm not sure it was teh right choice) and install it on your box.
Well, it may not be exactly finished. The tape I used proves to be too shiny for my taste, so there will be some light spilled via reflections from the grid itself (didn't actually checked how bad is it, but as you can see from the photo, the tape is only 2-3 stops darker than the diffusion material, when it should be as close to black as possible). I'm going to spray the whole thing with a coat or two of matte black paint before considering it completed. If I could find some thin sticky black velvet tape...
Sounds easy, but it's not. After a few rows I got better at this, now I can make a 10-cell row in about 15 minutes. My grid has 10 rows. Would I make another one? Well, maybe, but not anytime soon. I'd rather buy a ready-made grid instead - if only I could find one reasonably priced.
The grid seems sturdy enough to last for a while, and it rolls nicely to fit in the softbox storage case. But with the grid on it the soft box is about as heavy as the ballhead bracket can hold, I have to tighten that screw until I feel it's gonna' break.
See here another mod and a sample of the light pattern.