Hi, Ellie. If you have a friend with woodworking equipment they could make them real quickly, and probably out of scrap wood or cut-offs from other projects. You would need to let them know the size of the light base that will have to fit on them and the "angle" of the siding. To find the angle, get two rulers and a short level. Hold one at an even number of inches, like 4", 5" or 6", at the bottom edge of the siding and so the top edge of this ruler does not go up to the next piece of siding. Now put the level on the ruler edge and move it to center the bubble so it is straight up and down. Now measure the distance from the siding to the top of the ruler and be very exact. This will give you a slope like 5/16" in 4" or 7/32" in 5" or whatever yours measures. This will be easier if you have someone to help hold everything when you are measuring and be sure to write the numbers down.
Now you can give these two dimensions to your woodworking friend and he/she should be able to make you a block. Remind them to drill screw holes and a hole for the wiring with the drill press before they start shaping the wood. This will make it easier for you to mount the blocks as the holes will tend to hold the block level and not just crush the siding.
They might tell you that they cannot cut the slope angle with their table saw, and they are probably right. Remind them that they can do it on a planer if they use another piece of wood for a sled and a small block to hold the slope. But remind them also to watch out for the screws when they are planing. Planer blades are expensive. They can put the screws in a location that will be cut off later.
When you mount blocks, put them up once, being gentle so you don't crush the siding. Then take them off, use new screws, put paintable caulking on the back of the wood around the edges, and put them up again. When the caulking has set up in a day or so, apply a couple of coats of primer to the wood and then paint the same color as the siding. If your lights will be under the eaves, there is no reason to use pressure treated wood for this. Be sure that you have the wiring done safely also.
That should do it for you, if you can find the right person. If not, find a cabinet maker and they might do it for you too.