I'm going to tell you how I made a large handbag with a truly impressive volume — 16 800 cm3, which is 30 cm by 40 cm by 14 cm :)

I sew out of two types of felt — 2 mm and 1.2 mm. The bag will have pockets and a zip.
Draw the pattern, cut out its parts and felt details.

Here is a set of parts after cutting. Side walls, pockets, details of the top and bottom, the zip and a zip insert, handles:

Step 1. Handles.

Start with the handles. Draw a horizontal line at 4.5-5 cm from the rounded end. This is the markup for stitching.

Sew the handles in half up to the markup:

Insert a cord with the help of a sushi stick and a pin:

Glue the tips:

Make two handles:

Step 2. The top and the bottom

Sew the zip and a felt blank:

Sew them to the zip insert:

This is how it looks like:

Make straps for half rings:

And stitch them between the parts of the top and the bottom...

...that are then put together:

This is what you get:

Step 3. Pockets

First, insert a lock with a rotary mechanism.

A valve needs a ring, so first, sew two coloured halves together.

Mark a slit:

And cut it out:

Set ring, the valve is ready.

Stitch the smaller pockets along the top edge and insert the second part of the lock:

Turn the pocket inside out and slightly cut the grey part around the perimeter as in the photo:

Fasten the small pocket to a wall of a larger pocket:

Now join the parts of a large pocket:

Carefully stitch around the perimeter with a good allowance.

And slightly cut the inner part of the pocket from the inside.

Now cut out a hole to insert the valve. Lock it to stitch the valve correctly.

Stitch the perimeter of the hole with the valve. The pockets are ready.

Step 4. Final assembly

Baste the handles.

Join the bottom and the walls. I have calculated everything properly and the length of the top/bottom perfectly match the perimeter of the side walls.

Stitch the perimeter, treat the seams. Turn the bag outside in — and enjoy the result :)