I've made a USB dongle in transparent resin. The technique used is resin casting by using a silicon mould. That's the list of items and tools used:
-USB dongle, best a old one the first time, due it's a bit of risk to damage it.
-LEGO pieces to make a shuttering for the silicon mould.
-Modelling paste or any sculpture paste.
-Rubber for moulding, silicone. You can find it in some shop of toys, RC modelling, handcraft modelling or similar. I've bought it in a Spanish site called Feroca
-Transparent casting resin. Also you can find it in the same place than the silicone.
-Some system to measure the quantity of silicon, resin and catalyst. I've used a plastic glass and a syringe to calibrate the glass in ml and to add the catalyst.
-latex gloves to not touch the resin, the silicone rubber is easy to clean but the resin is like glue and smell very hard. You will need a very well ventilated place to use the resin.
-Vaseline to unmould the dongle
-Sand paper very thin, for a best finishing you will need a polish paste and a polish tool for your dremel.
-You could also insert some little pieces in the resin to make it more custom, I've added some little SMD resistors.

Let's go then..

Step 1: Preparing the Shuttering

First, make the shape of the USB dongle with the moulding paste.
Then you need to make a shuttering with the lego pieces that cover in height the dongle.
Put the dongle inside and fill the shuttering to the half with the paste.
Make four little spots of paste to make a union of the two parts of the mould.
Cover all, paste and shuttering inner sides with vaseline to unmould easily later
A plastic bag in the table will help to not dirt too much the work area, due the silicon rubber do not glue to it.

Step 2: Make the First Half

Calculate the volume of the shuttering and use the half (will be a little less) to do the rubber mix.
Add the catalyst in a 5% proportion or another if specified.
Fill the shuttering and wait for 24h or more, until the rubber is not sticky. Then unmould the rubber and clean the added modelling paste, you will have a half of the mould.

Step 3: Make the Second Half

You can unmount the lego pieces to clean better the added modelling paste, you will have what the picture shows without the dongle
Rebuild the shuttering, put again the dongle and vaseline over all.
Fill again with silicon rubber like in step 2.

Step 4: Mould Finished

After 24h more, you will have the two half's of the mould. Clean all very well and make a hole at the bottom of the mould to fill it with the resin

Step 5: Fill the Mould With Resin

Here I made a mistake, the resin have solvents and dissolve the plastic glasses, so use another thing like a crystal or ceramic glass to make the mix of the resin and the catalyst. Also uses the gloves and work in a very, very well ventilated room, this thing smells really bad
First calculate the volume of resin to use and make the mix with the catalyst. Put it in a half of the mould and wait until is like jelly (about 20 minutes), then you could insert what you want that will float on the resin.
Put the usb dongle in place, close with the other half of the mould and put it up to insert the rest of the resin by the back hole

Step 6: Finishing

After waiting 24h or more, depends of how correct you made the mix with the catalyst, unmould the dongle.
You will notice that is very irregular, due the modelling paste was not perfect flat. And also is not full transparent.
Then, you will need to use the fine sandpaper to make it more flat. For make it transparent I've used a polish paste for lens and a polish head for the dremel, so the surface is very clear.
And that's all folks.