Paint frosted glass panels

The T-square is just that - a T-square for making sure that the glass is cut square, unless it has an integral glass cutter of course.

The tool they use is a glass cutter. It will have a diamond wheel or tip and scores the glass where it needs to be cut. Then a sharp tap will (hopefully) produce a clean break.

They should be easy to pick up at any hardware store. You might have more difficulty getting hold of a large enough T-square, but for a cold-frame you don't necessarily have to be 100% accurate.

Glass can be quite expensive. It would be worth getting a quote for buying the glass you would need and then compare to the cost of the tools you need to buy and your time.

However if (as chris points out in his comment) the glass is tempered then you won't be able to cut it. It will simply shatter if you try.

If it is in a frame 1/4" glass will be sufficient. Your options for glass will be tempered glass or laminated glass because of it's proximity to the ground.

Tempered glass will break into very small pieces like a car window for a door. Tempered glass is stronger than normal glass.

Laminated glass will break like a car windshield. It breaks normally but it is two pieces bonded together with a plastic. The plastic prevents most injuries as well as the mess from breaking. Laminated glass is more often used for safety/security applications.

Although 1/4" is suitable for the application, I myself am always weary installing it as an interior partition. My family business has built all glass offices in buildings and we use 3/8" tempered glass. 3/8" Tempered is extremely difficult to break. When you get up to thicknesses that large as far as cost goes the difference between 3/8" and 1/2" thick glass isn't very significant.

If you go with a thicker size be sure that your trim is strong enough to withstand an impact of the weight of the glass, the weight of a person, and the velocity they are traveling. This is due to the fact that it is more likely to break from breaking out of its frame and falling down, especially in wood.

If the beams are not structural I would recommend you use a router to create a slot in each beam deep enough to slide the glass in far enough to place it where it needs to go and then slide it back towards the other beam into the slot you created there. This is commonly done with metal storefronts and is called a flush glaze.