As a little aside i decided to get a table saw or a bandsaw, BUT i cant afford one and my wife will kill me if i do buy one
So whats the solution..... MAKE ONE
You dont need to build big in this case the final surface area is 18inch square but use your own size it can be modified
Take your shelf or left over wood and mark out your cuts to make the working surface in my version as the shelf is 9 inches wide i need to make 2 planks 9x18
To make your joined work surface you need to use some more spare wood to make the rails on the underside, just cut your spare to match the length of the work surface and mark up a number of holes to screw the frame together
Clamp everything together tight, your final surface mush be perfectly flat
And here is how it all looks fixed together
With a set square mark up some lines so you can make sure you mount your saw perfectly in line with the sides
Drill holes for the saw blade to pass through you can have as many or as few as you want
Remove the base plate from your cheap jig saw ans drill some holes in it for your screw/bolt holes
Once you have that all put back together and have remounted your saw to its base and then to the work surface you can flip it over and mount it to your cheap work bench... this is where the split top in the bench comes in really handy to put the body of the jigsaw through, then just tighten your bench up it will grip the base of the saw and you are good to test it
i tried both free hand and with a guide i made later, i must admit free hand is a little scary you are afterall working with an upside down jigsaw but using a guide it is realy easy and the cuts are perfect. you will be able to see on the pic below which cut is free and which is with a guide