There is a way to create such a composite timber. Actually, all other things being equal (the species, cut and quality of the wood), a built up sandwich is actually stronger than a solid piece of wood.
This is done by laminating the three boards together. A generous layer of wood glue, such as Titebond II, is spread over one face of one of the boards. The next board is placed over this and a second spread of glue is placed on the face of board 2. Finally board three is placed on top.
The boards are aligned and clamped in numerous places along their length. If you are able to clamp them to a very firm perfectly flat surface (such as the platform of a table saw) even better. You need to make sure that there is no bow or twist in the clamped boards.
Once the glue dries (at least 24 hours) you can then trim the ends.
The newel post on the stair rail shown in this question and answer is made up from 3 1x3 oak boards laminated in the manner described.
Unless the table will bear an extremely heavy load or the taper is very extreme (or the wood you are using is balsa), a laminated 2.5 inch leg should be more than sufficient.
some glue that will set as rubber
That would be something like Sugru
Sugru is mouldable glue. Stick it, shape it and it will turn into rubber.
I don't know whether it will adhere well to aluminium and whatever polymer Apple use for "plastic" feet. However they do say:
Sugru sticks permanently to lots of stuff like ceramics, glass, metal, wood, and most plastics and fabrics.
(my emphasis)
The MBP gets hot
According to Sugru:
it'll stay strong and securely bonded anywhere from the freezer to a steamy hot shower
Might be worth trying.
Best Answer
It isn't clear what the materials are. Some of the likely materials are very difficult to bond; even tougher if it is two different hard-to-bond materials. It is also an application where there will be a lot of stress and distortion, which is especially challenging.
If you really want to try it, one thing that might work is to heat-weld the two using a third material that bonds with both. For example, there are plastic welding sticks like this that are supposed to bond with virtually any type of plastic, including polyethylene, polypropylene, and hard rubber-like materials, which are common materials for those kinds of glides.
They're used for tasks like repairing plastic/rubber car bumpers. If you don't have a plastic welder, you can use a soldering iron if you're careful to not overheat the materials.
That said, A better solution would be a single glide that does what you want, or wheels rather than a glide. Also look at plastic chair mats that sit on the carpet. They protect the carpet from being crushed and make it easy to move the chair.