Variant 1: random

Like UUIDv4: all 100 payload bits random.

Layout

1111.1111.1111.1111.1111.2222.2222.2222-
2222.2222.2222.2222-
3333.2222.2222.2222-
4455.2222.2222.2222-
2222.2222.2222.2222.2222.2222.2222.2222.2222.2222.2222.2222

 0                   1                   2                   3
 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
|                  name                 |         random        |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
|             random            |  ver  |         random        |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
|var| tv|                         random                        |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
|                             random                            |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

#	Field	Bits	MSB = 0	Value
1	Name	20	0–19	Name encoding
2	Random	100 = 28 + 12 + 60	20–47, 52–63, 68–127
3	UUID version	4	48–51	`0x8`
4	UUID variant	2	64–65	MUST be `0b10`
5	TNID variant	2	66–67	MUST be `0b01`

The spec names no random source. Use a CSPRNG wherever guessing a TNID could cause harm.

Goals and non-goals

Goals: maximum entropy within the UUID and TNID specs; reasonable for 99% of use cases (low chance of collision, low stakes if one happens).
Non-goal: maximum entropy at the expense of TNID's other goals.

Caveats

Fewer random bits than UUIDv4

100 random bits vs UUIDv4's 122: the name takes 20, the TNID variant 2. Only TNIDs with the same name can collide. Birthday bound, one name:

TNIDs	Chance of any collision
10⁹	About 1 in 2.5 trillion
≈ 1.3 × 10¹⁵	50%